Tottenham boss Mauricio Pochettino blasted VAR after their FA Cup win over Swansea City.
Spurs were 1-0 up in their FA Cup quarter-final when Heung-Min Son had a 23rd-minute goal ruled out for offside.
The incident was reviewed at length by VAR but despite delaying the game for several minutes the footage proving inconclusive, so the original decision was upheld.
Pochettino said: "It's a nightmare. I feel so sorry for the people trying to use that system.
"I think I prefer it when the ref and assistant make mistakes, than to wait three or four minutes for things.
"But we now need to see the reality of how it is going to affect the staff on the touchline and the officials.
"It is so complex, but the worst thing for me is the effect for the fan. It is going to be a massive problem for the future."